Fix probelms with DTLS when no packets are pending.

When using DTLS you might run into the situation where no packets
are pending, so SSL_read returns len=0. On a TLS connection this
means that the connection was closed, but on DTLS it does not
(a DTLS connection cannot be closed in the usual sense).
This commit fixes a bug introduced by c8d23f37.

Conflicts:
	openssl/src/ssl/mod.rs